1. Field of the Invention
The present invention relates to a developing device for use in a copying machine, printer, facsimile machine, or the like that employs electrophotographic process or electrostatic recording.
2. Related Background Art
According to the method commonly used in multicolor image formation, toner images in different colors from one another are sequentially formed on a photosensitive member and the toner images in different colors are overlapped on a transfer medium such as a sheet of paper sequentially or collectively. For such multicolor image forming apparatus, a so-called rotary developing process has been proposed and put into practical use. In this process, developing units for black, yellow, magenta, and cyan colors are mounted to a rotary developing member and the rotary developing member is rotated to sequentially deliver necessary developing units to a development position facing a photosensitive member that is an image bearing member for a development operation.
On the other hand, dual-component development in which non-magnetic toner and a magnetic carrier are mixed for use as developer is widely employed among conventional electrophotographic process image forming apparatus, especially multicolor image forming apparatus for forming chromatic color images. Compared to other development methods that have been proposed, dual-component development has advantages including stable image quality and apparatus durability. However, long use means that developer degradation, carrier degradation in particular, is unavoidable. Replacing old developer with new one is therefore necessary for multicolor image forming apparatus that is long in use and it results in an increase in service cost or running cost.
Several solutions have been proposed for this problem. One of these solutions is a developing unit that is loaded with a replaceable developer replenishing cartridge to replenish developer containing toner and a carrier and to collect the developer (see JP 06-308829 A).
However, this developing unit makes image forming apparatus large in size and its control mechanism complicated because it employs a structure in which plural screws are used to collect developer that has overflown the developing unit and send the developer to the developer replenishing cartridge. With the thus structured developing unit and developer replenishing cartridge mounted, a rotary developing member itself is increased in diameter and the multicolor image forming apparatus becomes large in size. Furthermore, the developer feeding path is complicated enough to cause leakage of the developer when the rotary developing member is rotated.
Then, a developer discharging method has been put into practical use particularly for the rotary developing process. This method utilizes changes in direction the gravity works which accompany the rotational motion of the rotary developing member to replenish a developing unit with dual-component developer and to discharge the dual-component developer from the developing unit (JP 09-218575 A).
On the other hand, JP 10-142888 A discloses another structure: at a development position where a developing unit faces a photosensitive drum, developer discharged from a developing container is temporarily pooled in a reservoir, is fed to a cylindrical shaft at the center of a rotary developing member utilizing a change in direction the gravity works due to rotation of the rotary developing member, and ultimately is collected by a developer feeding member in the cylindrical shaft to be sent into a developer collecting container that is provided at a shaft end of the cylindrical shaft. Similar to the multicolor image forming apparatus proposed in JP 09-218575 A, this structure discharges developer by utilizing the motion unique to any rotary developing member. Therefore, this structure prevents carriers from losing their charging ability while avoiding an increase in size of the multicolor image forming apparatus. In addition, even when monochromatic images are formed in succession, excess developer in the developing unit is discharged and sent into the reservoir outside of the developing unit at the development position without stopping the development operation. Accordingly, the image productivity is not lowered and the developer amount in the developing unit is kept within an acceptable range.
JP 11-249418 A presents an example in which a feeding unit for feeding excess developer to a cylindrical shaft at the center of a rotary developing member is structured to be collectable utilizing a change in gravitational direction due to rotation of the rotary developing member. In this proposal, a spiral auger is arranged in the interior of the cylindrical shaft that is provided so as to rotate with the rotary developing member as one, and the spiral auger is rotated and driven by a drive motor to feed excess developer in the cylindrical shaft.
These examples of the conventional art have the following problems.
To meet the demand made along the recent growth of full-color copying machine/printer market for various functions, a lot of multicolor image forming apparatus reduced in size and cost while aiming at high image productivity have been commercialized and they are expected to rank among the mainstream of the future market.
In this context, it is apparent that techniques related to a simply-structured developing unit which prevents carriers from losing their charging ability without increasing the size or cost of multi-color image forming apparatus, which suppresses an increase in service cost or running cost, and which replenishes developer containing toner and a carrier as well as collects excess dual-component developer, will continue to hold an important position.
The method of discharging developer to the outside of a rotary developing member after excess developer is collected in the center of the rotary developing member collectively as shown in the examples of the conventional art (JP 10-142888 A and JP 11-249418 A), allows degraded excess developer in a developing unit to gather in one place and does not need to process degraded excess developer in individual developing units separately. This method is therefore desired to improve through technical development in the future.
However, the multicolor image forming apparatus of JP 10-142888 A and the multicolor image forming apparatus of JP 11-249418 A both employ the structure in which an external input is used to rotate and drive screws arranged in a cylindrical shaft that is located in the vicinity of the central axis inside the rotary developing member. This complicates the drive system of the image forming apparatus main body and the cost is raised because of drive trains.
Specifically, laborious installment of the complicated drive system and an increase in number of parts raise cost. The drive system is complicated because it is necessary to input a driving force from a development drive motor into the feeding screws by connecting the drive trains from the drive system in order to rotate the feeding screws during a continuous image-forming operation, which is most appropriate considering that excess developer is discharged in the cylindrical shaft portion during development.
Another method conceivable is to input a driving force to the feeding screws by splitting a drive system for rotating and driving a rotary developing member. A rotary motor for the rotary developing member rotates and drives at a high speed the rotary developing member, which is very large in inertial mass. Therefore, in most cases, a large-sized stepping motor is used as the rotary motor and the driving force is transmitted directly to a large-diameter gear of the rotary developing member with the reduction gear ratio set extremely low. Driving the feeding screws by the driving force from this drive system often presents design difficulties. Accordingly, complication of image forming apparatus and an increase in cost is unavoidable in implementation of this method.
Then, the method disclosed in an embodiment of JP 11-249418 A, which is above-described as an example of the conventional art and in which the rotational motion of a rotary developing member is utilized to discharge from its central cylindrical shaft degraded excess developer to the outside of the rotary developing member, has attracted attention as a simple, effective technique which does not cause enlargement of multicolor image forming apparatus or an increase in cost. However, the multicolor image forming apparatus disclosed in JP 10-142888 A and the multicolor image forming apparatus disclosed in JP 11-249418 A both need to place an excess developer collecting inlet of the cylindrical shaft on the cylinder surface in order to collect excess developer. If a helical groove is formed on the inner wall of the cylindrical shaft and is rotated together with the rotary developing member, the collecting inlet too is rotated to make collecting excess developer impossible.
Rotating a cylindrical shaft of a rotary developing member is also necessary in the method of feeding excess developer through the rotational motion of a rotary developing member by fixing an end of a feeding screw to a drive unit that is provided on a side plate of the main body. In addition, the ability of feeding excess developer is insufficient. These feeding methods that utilize the rotational motion of a rotary developing member are therefore cannot be employed in real products.